Optical integrator for an illumination system of a microlithographic projection exposure apparatus

ABSTRACT

The disclosure relates to an optical integrator configured to produce a plurality of secondary light sources in an illumination system of a microlithographic projection exposure apparatus. The disclosure also relates to a method of manufacturing an array of elongated microlenses for use in such an illumination system. Arrays of elongated microlenses are often contained in optical integrators or scattering plates of such illumination systems.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/186,365, filed Aug. 5, 2008, which is a continuation of internationalpatent application serial number PCT/EP2007/001370, filed Feb. 16, 2007,which claims benefit of U.S. provisional patent application Ser. No.60/774,850 filed Feb. 17, 2006 and U.S. provisional patent applicationSer. No. 60/804,369, filed Jun. 9, 2006. The full disclosure of theseapplications is incorporated herein by reference.

FIELD

The disclosure relates to an optical integrator configured to produce aplurality of secondary light sources in an illumination system of amicrolithographic projection exposure apparatus. The disclosure alsorelates to a method of manufacturing an array of elongated microlensesfor use in such an illumination system. Arrays of elongated microlensesare often contained in optical integrators or scattering plates of suchillumination systems.

BACKGROUND

Microlithography (also called photolithography or simply lithography) isa technology for the fabrication of integrated circuits, liquid crystaldisplays and other micro-structured devices. The process ofmicrolithography, in conjunction with the process of etching, is used topattern features in thin film stacks that have been formed on asubstrate, for example a silicon wafer. At each layer of thefabrication, the wafer is first coated with a photoresist which is amaterial that is sensitive to radiation, such as deep ultraviolet (DUV)light. Next, the wafer with the photoresist on top is exposed toprojection light through a mask in a projection exposure apparatus. Themask contains a circuit pattern to be projected onto the photoresist.After exposure the photoresist is developed to produce an imagecorresponding to the circuit pattern contained in the mask. Then an etchprocess transfers the circuit pattern into the thin film stacks on thewafer. Finally, the photoresist is removed. Repetition of this processwith different masks results in a multi-layered microstructuredcomponent.

A projection exposure apparatus typically includes an illuminationsystem, a mask stage for a aligning the mask, a projection lens and awafer alignment stage for aligning the wafer coated with thephotoresist. The illumination system illuminates a field on the maskthat often has the shape of an (elongated) rectangle or a ring segment.

In current projection exposure apparatus a distinction can be madebetween two different types of apparatus. In one type each targetportion on the wafer is irradiated by exposing the entire mask patternonto the target portion in one go; such an apparatus is commonlyreferred to as a wafer stepper. In the other type of apparatus, which iscommonly referred to as a step-and-scan apparatus or scanner, eachtarget portion is irradiated by progressively scanning the mask patternunder the projection light beam in a given reference direction whilesynchronously scanning the substrate parallel or anti-parallel to thisdirection. The ratio of the velocity of the wafer and the velocity ofthe mask is equal to the magnification of the projection lens, which isusually smaller than 1, for example 1:4.

SUMMARY

In some embodiments, the disclosure provides an optical integrator forproducing a plurality of secondary light sources in an illuminationsystem of a microlithographic projection exposure apparatus. The opticalintegrator can make it possible to achieve a desired irradiance andangular far field distribution in a mask plane. More particularly, auniform or desired nonuniform irradiance distribution can be achieved inwhich undesired ripples are reduced.

In certain embodiments, the disclosure provides a method formanufacturing very accurately an array of elongated microlenses for usein an illumination system in a microlithographic projection exposureapparatus.

In some embodiments, an optical integrator includes a first array ofelongated convexly curved first microlenses that are arranged side byside in a first plane and have first vertex lines. The opticalintegrator further includes a second array of elongated convexly curvedsecond microlenses that are arranged side by side in a second plane andhave second vertex lines. At least one second vertex line or a portionthereof does not coincide, in a projection along an optical axis of theoptical integrator, with anyone of the first vertex lines or portionsthereof.

It has been discovered that by laterally displacing correspondingmicrolenses it is possible to laterally shift also the intensitydistribution in the far field. If the vertex lines of a plurality ofcorresponding microlenses do not coincide in a projection along theoptical axis, the laterally displaced intensity distributions in the farfield superimpose to a total intensity distribution in which ripplespresent in each intensity distribution produced by a single microlensare reduced as a result of an averaging effect.

There are various approaches how a complete coincidence of the vertexlines of all or some pairs of corresponding microlenses can be avoided.For example, the at least one second vertex line may form a zigzag or aserpentine line. In the case of a zigzag line adjacent straight portionsof the at least one second vertex line may be inclined by a common angleδ with respect to a reference direction. This ensures that the distancesby which the intensity distributions in the far field are laterallydisplaced follow an equipartition function which ensures the bestaveraging effect.

As a matter of course, the vertex lines of both the first and the secondmicrolenses may form zigzag or serpentine lines, or only portions withinthe first and/or second microlenses may have vertex lines that do notcoincide with the vertex line of the corresponding microlens in theother array. It should be noted that the light may propagate through theoptical integrator either along a direction from the first to the secondmicrolenses or along the opposite direction.

Instead of or in addition to the introduction of an inclination anglebetween the vertex lines of corresponding microlenses it may also beenvisaged to provide second microlenses having varying pitches. Acomplete coincidence between corresponding vertex lines is then avoidedeven if both the first and the second vertex lines are straight.

In certain embodiments, an optical integrator includes a first array ofelongated first microlenses that are arranged side by side in a firstplane. The optical integrator further includes a second array ofelongated second microlenses that are arranged side by side in a secondplane. At least one second microlens has a curved surface profile whichvaries along a longitudinal axis of the at least one second microlens.

Such a variation of the curved surface profile of microlenses makes itpossible to change the angular distribution produced by the microlensesin certain areas distributed over an exit surface of the opticalintegrator. For example, it is possible to introduce a field dependentvariation of the telecentricity and ellipticity properties of theprojection light impinging on the mask plane.

A local change of the curved surface profile may also be advantageous toreduce ripples in the far field intensity distribution, becausedifferent curved surface profiles produce different far field intensitydistributions that result, if superimposed in a mask plane, in asmoother overall intensity distribution. In this case it may be desiredto restrict the surface profile variation to individual microlenses.

For modifying the telecentricity and ellipticity properties in the farfield, it is usually desired to provide a plurality of adjacent secondmicrolenses that have curved surface profiles which vary alonglongitudinal axes of the plurality of second microlenses. For example, asecond array of second microlenses may include at least one first areain which the second microlenses have first curved surface profiles. Inat least one second area the second microlenses have second curvedsurface profiles that differ from the first curved surface profiles inthat material has been locally removed. If material is removed only inthe at least one second area, for example by polishing or ion beametching, the curved surface profiles are locally modified which resultsin a field dependent change of the telecentricity and ellipticityproperties of the illumination system. Before the material is removed,the second curved surface profiles may all be identical or may differfrom each other, for example with respect to the radius of curvature orthe microlens pitch.

For modifying the telecentricity and ellipticity properties of theillumination system, at least two second areas may be arranged along afirst direction, and at least two further second areas may be arrangedalong a second direction which is at least substantially perpendicularto the first direction. The second microlenses in the second areasarranged along the first direction then have curved surface profilesthat are different from the curved surface profiles of the secondmicrolenses in the further second areas arranged along the seconddirection.

In some embodiments, the disclosure provides a method that includesproviding a substrate and a cutting tool including a cutting edge. Thecutting tool is repeatedly moved relative to the substrate in a fly-cutprocess, thereby enabling the cutting edge to cut into the substrate.During this step the substrate is moved along a longitudinal directionwhich is parallel to the longitudinal axes of the microlenses. Thesubstrate is then moved at least substantially perpendicular to thelongitudinal direction. Then the steps of moving the cutting tool andsimultaneously moving the substrate are repeated until all microlensesare formed on the substrate.

In principle, it is possible to form microlens arrays also on curvedsurfaces. However, it will usually be desired to move the substrateparallel to a machining plane. If the substrate is moved during thefly-cut process along straight parallel lines, cylindrical microlensesare formed which are all arranged in parallel. If the substrate is movedalong zigzag or serpentine lines, the longitudinal axes of themicrolenses will also have the shape of zigzag or serpentine lines.

During the fly-cut process the cutting tool is optionally rotated arounda rotational axis. This rotational axis may extend at leastsubstantially parallel to a substrate surface on which the microlensesare to be formed. In a fly-cut process the cutting edge points away fromthe rotational axis and may be spaced apart from the rotational axis byat least 5 mm, such as by more than 9 mm and less than 11 mm. With suchdistances the rotational frequency of the cutting tool may be in excessof 5000 1/min, such as between 8000 1/min and 10000 1/min.

If the substrate is, between the formation of single microlenses,laterally moved substantially by a multiple of the pitch of themicrolenses, an interleaved microlens manufacturing sequence may beobtained. This may be advantageous because the cutting edge is usuallysubject to wear, and consequently the microlenses formed during thefly-cut process will have slight variations of their surface profile. Bythe application of an interleaved manufacturing sequence, the variationsare distributed over the entire surface of the substrate, which has apositive effect on the uniformity of the far field intensitydistribution.

For changing a surface profile of at least one microlens, at least aportion of the surface of this microlens may be reworked. This may beaccomplished, for example, by repeating the fly-cut process with acutting tool having a different cutting edge. A cutting edge havingdifferent cutting properties may be obtained by simply turning one andthe same cutting tool by 180°.

The surface may reworked also by polishing the at least one microlens,or by exposing the at least one microlens or at least a portion thereofto an ion beam. The ion beam should have a cross section with a maximumdimension which is greater than a pitch of the at least one microlens. Aplurality of microlenses is then exposed to the same flow of ionsimpinging in parallel on the surface of the microlens. Since the angleformed between the ion beam direction and the surface strongly varies incross sections perpendicular to the longitudinal directions of themicrolenses, the etch rate also strongly varies within the crosssections. This makes it possible to modify the curved surface profilesof the microlenses with a homogenous ion beam, without the need todirect a very thin ion beam on a portion of a single microlens, and tovary the etch rate by controlling the etch time. For influencing theetch rate, the orientation between the at least one microlens or aportion thereof and the ion beam may be changed, for example by using atilt table on which the substrate with the microlens array is attached.

Alternatively or additionally, a coating consisting of a carefullyselected material, or including layers of different materials, may bedeposited on the microlenses. The material of the coating then providesan additional degree of design freedom that may be exploited when theprocess parameters are determined that are used for obtaining thedesired surface manipulation. If the material is an anisotropic crystalsuch as CaF₂, the etch rate R also depends on the crystal orientation.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and advantages of the present disclosure may be morereadily understood with reference to the following detailed descriptiontaken in conjunction with the accompanying drawing in which:

FIG. 1 shows a perspective view of an optical integrator;

FIG. 2 is a schematic meridional section of some microlenses of theoptical integrator, a condenser lens and a mask plane of an illuminationsystem of a microlithographic projection exposure apparatus;

FIG. 3 is a schematic side view of a machine for manufacturing theoptical integrator shown in

FIG. 1 using a fly-cut process;

FIG. 4 is a side view of a holder for a tool used for the fly-cutprocess;

FIG. 5 is a perspective view of the tool;

FIG. 6 is a top view of the tool;

FIG. 7 is a top view on a (111) CaF2 substrate;

FIGS. 8 and 9 show the crystal orientation of the substrate shown inFIG. 7 for microlens arrays on opposite sides of a substrate;

FIG. 10 is a top view on a microlens array with areas in which thesurface profiles of the microlenses differ, which results in intensitydistributions indicated in circles;

FIG. 11 is a graph showing the dependency of the etch rate from theangle of incidence for ion beam edging;

FIG. 12 is a perspective schematic illustration of a general setup forion beam edging of microlenses;

FIG. 13 illustrates the term azimuth angle and aperture angle;

FIG. 14 shows another setup for ion beam edging in a representationsimilar to FIG. 11, but with an azimuth angle β=0°;

FIG. 15 shows a graph illustrating the dependency of the etch rate Rfrom the aperture angle and the distance from the vertex line of themicrolens;

FIG. 16 shows another setup for ion beam edging in a representationsimilar to FIG. 12, but with an azimuth angle β=90°;

FIG. 17 is a graph similar to FIG. 15 for the setup shown in FIG. 16;

FIG. 18 is a graph similar to FIG. 17, but with ion beams directedsymmetrically to both sides of the microlenses;

FIG. 19 is a side view of two opposing microlens arrays, wherein themicrolenses of one array has varying pitches;

FIG. 20 is a graph showing the intensity distribution in the mask planeproduced by a single microlens;

FIG. 21 is a graph showing the total intensity distribution in the maskplane obtained by the superposition of the intensity distributions ofall microlenses of the microlens array shown in FIG. 19;

FIGS. 22 a to 22 l illustrate the manufacture of microlenses having avarying pitch with a single cutting tool;

FIG. 23 is a top view of a microlens array in which the longitudinalaxes of the microlenses are parallel to a zigzag line;

FIG. 24 is a side view of the microlens shown in FIG. 23;

FIGS. 25 a to 25 e illustrate the manufacture of a microlens array withan interleaving microlens formation;

FIG. 26 is a side view of a microlens array illustrating the reworkprocess;

FIG. 27 is a side view similar to FIG. 26;

FIG. 28 is a section through a microlens array in which the microlenseshave surface profiles with different circular curvatures;

FIG. 29 is a schematic front view of a multi-edge cutting tool;

FIG. 30 is a front view of a multi-edge cutting tool;

FIG. 31 is a perspective view of an optical arrangement including theoptical integrator shown in FIG. 1 and two scattering plates.

DETAILED DESCRIPTION

1. Optical Integrator

In the following the general structure and function of an opticalintegrator in accordance with the present invention will be explained.

1.1 General Structure

FIG. 1 is a simplified and perspective view of an optical integratorwhich is denoted in its entirety by 10. The optical integrator 10 ofthis embodiment is made up of a first integrator member 12 and a secondintegrator member 14. The first integrator member 12 includes a firstarray of cylindrical microlenses 12Y having parallel longitudinal axesthat are aligned along an X direction. The first integrator member 12further includes an array of second cylindrical microlenses 12X thathave parallel longitudinal axes aligned along a Y direction which isperpendicular to the X direction. Since the longitudinal axes of themicrolenses 12Y, 12X are straight, the first microlenses 12Y have arefractive power only along the Y direction, and the second microlenses12X have a refractive power only in the X direction.

The second integrator member 14 is an identical copy of the firstintegrator member 14, but is mounted after rotation by 180° aroundeither the X or the Y axis. Thus third microlenses 14X are facing thesecond microlenses 12X, and fourth microlenses 14Y are facing away fromthe first integrator member 12.

In the embodiment shown the focal lengths of the microlenses 12X, 12Y,14X, 14Y and the distance between the integrator members 12, 14 alongthe optical axis OA are selected such that focal lines produced by thesecond microlenses 12X are located on the vertices of the thirdmicrolenses 14X. Since the third microlenses 14X have the same focallength as the second microlenses 12X, this implies that the focal linesof the third microlenses 14X are located on the vertices of the secondmicrolenses 12X. The same conditions also apply, mutatis mutandis, tothe first and fourth microlenses 12Y and 14Y, respectively.

In an alternative embodiment (not shown), the second and thirdmicrolenses 12X and 14X, respectively, are different. More particularly,the focal length of the second microlenses 12X is greater than the focallength of the third microlenses 14X. The distance between the integratormembers 12, 14 may then be selected such that the focal lines of thethird microlenses 14 are still on or in close proximity to the verticesof the second microlenses 12X. The focal lines of the second microlenses12X are within the fourth microlenses 14Y or even further away, i.e.outside the second integrator member 14.

Each microlens 12Y, 12X, 14X, 14Y is geometrically defined by itslongitudinal axis, its pitch, i.e. the width perpendicular to itslongitudinal axis, and the curved surface profile of the refractivesurface. The term “profile” relates to the shape of the surface in (any)cross section perpendicular to the longitudinal axis of the microlens.

The surface profile mainly determines the optical properties of themicrolenses. For example, if the second microlenses 12X have curvedsurface profiles which are circular, the second microlenses 12X have ina XZ plane the effect of spherical lenses. If the curved surfaceprofiles are not circular, the second microlenses 12X have in this planethe effect of aspherical lenses.

The pitches of the microlenses 12Y, 12X, 14X, 14Y should be below 5 mmor better below 2 mm. In the embodiment shown the pitches are in a rangebetween several micrometers up to several hundred micrometers, forexample 500 μm. Usually it is desired to have pitches that are greater,for example by a factor 3, than the wavelength of the light whichpropagates through the optical integrator 10.

Various other alternative structures of optical integrators aredescribed in International Patent Application No. PCT/EP2007/001267,which claims priority to the same U.S. provisional application60/774,850 filed Feb. 17, 2006 as is claimed by the present application.For example, the first, second, third and fourth microlenses 12Y, 12X,14X, 14Y may be distributed to more than two integrator members, andalso the sequence of the microlenses may be changed.

1.2 Function

The optical integrator 10 is intended for producing a plurality ofsecondary light sources in an illumination system of a microlithographicprojection exposure apparatus. Each secondary light source completelyilluminates the illuminated field on the mask. This will be explainedwith reference to FIG. 2; a more detailed explanation of the functioncan be found in the aforementioned International Patent Application No.PCT/EP2007/001267.

FIG. 2 shows three third microlenses 14X, a condenser lens 16 of anillumination system and a mask plane 18 (or an intermediate field plane)which is to be illuminated by the optical integrator 10. Each microlens14X is illuminated by a corresponding second microlens 12X and producesa divergent light bundle. The angular intensity distribution of thelight bundles is determined by the curved surface profile of themicrolenses 14X. The condenser lens 16 transforms the angulardistribution into an intensity distribution in the mask plane 18.

In projection exposure apparatus of the scanning type, the illuminatedfield on the mask has usually large dimensions along an X direction andmuch smaller dimensions along a Y direction. This has also implicationsfor the refractive power of the microlenses 12Y, 12X, 14X and 14Y. Thefirst microlenses 12Y and the fourth microlenses 14Y require only asmall refractive power because the illuminated field has a smalldimension along the Y direction. The second microlenses 12X and inparticular the third microlenses 14X desirably have a larger refractivepower because they have to produce light bundles having a larger maximumdivergence angle.

Ideally the angular distribution is perfectly smooth, and each microlens14X illuminates the mask plane 18 with a perfectly homogeneous oruniform intensity distribution. If the surface profile of themicrolenses does not correspond to its ideal shape, for example as aresult of a significant surface roughness, the angular distribution willnot be perfectly smooth, and consequently ripples will be observed inthe mask plane 18. Since the intensity distributions produced by allmicrolenses 14X are superposed in the mask plane 18, the ripples may besmoothed as a result of the averaging effect which is associated withthe superposition of a large number of intensity distributions. Thisrequires, however, that the intensity distributions produced by eachmicrolens 14X are not perfectly identical. Usually the manufacturingprocess of the microlenses 14X ensures that there are certain variationsbetween the microlenses. However, these variations may not besufficient.

The optical integrator 10 is characterized by the introduction ofdesigned, i.e. deliberately introduced, variations. Namely, the thirdmicrolenses 14X or different portions within the third microlensesdiffer from one another with respect to the pitch and/or thelongitudinal axis and/or the curved surface profile. This will beexplained in more detail further below under section 3.3.

2. Manufacture of Optical Integrator

In the following a method of manufacturing the optical integrator 10will be explained with reference to FIGS. 3 to 16. The method caninvolve a fly-cut process that is carried out on a fly-cut machine.

2.1 General Outline of Fly-Cut Machine

FIG. 3 is a schematic side view of a fly-cut machine which is denoted inits entirety by 20. For the sake of clarity the illustration of FIG. 3is not to scale.

The machine 20 is used to produce the microlenses 12Y, 12X, 14X, 14Y ona suitable substrate by way of a fly-cut process. A fly-cut processdiffers from a conventional turning process, in which a workpiece isrotated relative to a slowly moving tool, in that the workpiece isslowly moving and the tool is rotated or quickly moved in another way.

For realizing a fly-cut process the fly-cut machine 20 includes acutting tool 22 with a cutting edge 24. The cutting tool 22 is attachedto a holder 25, which is, in the embodiment shown, adjustably mounted toa spindle 26 which is rotated around a rotational axis 28. In general,the cutting edge 24 is spaced apart from the rotational axis 28 by atleast 5 mm, such as by more than 9 mm and less than 11 mm. With atypical rotational frequency in excess of 5000 1/min (e.g., between 80001/min and 10000 1/min), the cutting edge 24 is moved with a very highvelocity relative to a substrate denoted by 30. This makes it possibleto produce very smooth and accurate surface profiles on a slowly movingsubstrate 30.

The substrate to be machined is received in a workpiece holder 32 whichis attached via an adaptor member 34 to a lift table 36. The lift table36 enables the substrate 30 to be adjusted relative to the cutting edge24 along a Z direction which usually coincides with the verticaldirection.

The lift table 36 is mounted to a tilt table 38 which makes it possibleto tilt the substrate 30 around two orthogonal X and Y directions, as isindicated in FIG. 3 by arrows 40 and 42. The tilt table 38 is mounted ona turn table 44 that makes it possible to rotate the substrate 30 arounda rotational axis 46 (see double arrow 48), which can intersect therotational axis 28 of the spindle 26. The turn table 44 is mounted on apositioning table 50 that makes it possible to displace the substrate 30along orthogonal X and Y directions in a plane perpendicular to the Zdirection. The positioning table 50 may have a positioning accuracy of10 nm or even better relative to the spindle 26 and thus relative to therotating cutting edge 24.

The superposition of the various movements produced by the lift table36, the tilt table 38, the turn table 44 and the positioning table 50makes it possible to produce virtually any arbitrary movement of thesubstrate relative to the fast rotating cutting edge 24. This, in turn,makes it possible to produce a wide variety of structures on a substratethat even may have irregular shapes.

FIG. 4 is a schematic side view of the holder 25 including a shaft 52 towhich the cutting tool 22 is attached, for example by soldering. Theshaft 52 can be precisely adjustable with respect to the spindle 26 inorder to achieve a correct orientation of the tool 22 with respect tothe rotational axis 28 of the spindle 26. Alternatively or additionally,an adjustment mechanism may be provided between the tool 22 and theshaft 52.

FIGS. 5 and 6 show the tool 22 in a perspective view and a top view,respectively. The tool 22 is basically crescent-shaped, with the cuttingedge 24 being formed on the inner side of the crescent.

As becomes clear from the top view of FIG. 6, the tool 22 has a largerfront surface 54 on which the cutting edge 24 is formed, and a smallerback surface 56 so that the tool 22 recedes behind the cutting edge 24.The curvature of the cutting edge 24, which determines the surfaceprofile of the microlenses to be produced, is smaller than the curvatureof the adjacent curved edge 58 of the back surface 56. During thefly-cut process, the rotational direction of the spindle 26 (see arrow50) is selected such that the front surface 54 with the cutting edge 24moves towards the substrate, and the cutting edge 24 is first to cutinto the surface of the substrate. The adjacent curved edge 58 usuallydoes not come into contact with the substrate during the fly-cutprocess.

In the embodiment shown the cutting edge 24 defines a concave arch of acircle. With such a cutting edge 24 cylindrical convexly curvedmicrolenses can be manufactured in a fly-cut process that have, in aplane perpendicular to the longitudinal axes of the microlenses, theeffect of spherical lenses. The width W between the ends of the cuttingedge 24 defines the maximum pitch of the microlenses that may beproduced with the tool 22. In the embodiment shown the width W equals1.500 mm, with an accuracy of 0.001 mm.

The tool 22 is can be made of a monocrystalline diamond. Depending onthe material of the substrate, other materials, such as conventionaldiamonds, a hard metal or ceramics, may be envisaged as well.

2.2 Manufacturing Process

In the following the fly-cut process using the fly-cut machine 20 willbe briefly described.

First the substrate, which can have the shape of a plane-parallel plate,is mounted to the workpiece holder 32 and adjusted using the tilt table38 such that its upper surface pointing towards the spindle 26 isexactly perpendicular to the Z direction. The lift table 36 is used todisplace the substrate 30 along the Z direction such that the cuttingedge 24 cuts completely into the surface of the substrate 30. Duringrotation of the spindle 26 the substrate 30 is slowly moved, with thehelp of the positioning table 50, along a first direction which definesthe longitudinal axes of the microlenses to be manufactured. Thisfly-cut process produces, with a tool as shown in FIGS. 5 and 6, a firstconvexly shaped cylindrical microlens having a straight longitudinalaxis.

If the desired length of the microlenses is achieved, the lift table 36lowers the substrate 30, and the positioning table 50 displaces thesubstrate 30 in a transverse direction before the lift table 36 liftsthe substrate 30 again. By moving the substrate 30 slowly again, but nowopposite to the first direction, a second microlens is produced whichextends parallel to the first microlens that has been produced before.If a more homogeneous surface of the microlenses is desired, movementsof the substrate 30 during the fly-cut process may not only be parallel,but also be pointed towards the same direction. This is because it maymake a difference to cut the cutting edge 24 in a substrate 30 thatmoves in a same direction as the cutting edge 24 or in an oppositedirection.

This process is repeated until the desired number of parallelmicrolenses is produced on the substrate 30. If microlenses shall alsobe manufactured on the opposite side of the substrate 30, as is the casewith the optical integrator members 12 and 14, the substrate is thenreleased from the workpiece holder 32, reversed and fixed in thisreversed orientation in the workpiece holder 32. The process is thenrepeated. For producing two orthogonal arrays of microlenses on oppositesides of the substrate 30, the slow movements of the substrate duringthe fly-cut process on both sides are orthogonal.

Instead of moving the substrate 30 along the Z axis with the help of thelift table 36, the spindle 26 may be connected to a modulating unit thatmakes it possible to move the spindle 26, and thus the cutting edge 24,along the Z direction by small, but very precisely controllabledistances.

2.3 General Properties of the Fly-Cut Process

Since the fly-cut process does not involve an etch step, as is the casewith some of the conventional methods of producing microlenses, alsomicrolenses made of CaF₂ can be manufactured. This is a significantadvantage because CaF₂ is a very important optical material for opticalintegrators used at wavelengths below 200 nm.

The fly-cut process also makes it possible to produce microlenses havingan excellent edge sharpness. This is particularly important withmicrolens arrays in which the adjacent microlenses are in immediatecontact. As can be seen in the enlarged cutout of FIG. 12, adjacentmicrolenses abut along straight edges 62 where the curvature is, in theideal case, zero. However, due to the good edge sharpness that may beachieved with the fly-cut process, it is possible to approximate theideal shape along the edges 62 to a very good extent. Accordingly, arrayareas which are optically not usable or that re-fract light to undesireddirections can be kept very small.

The edge sharpness may be defined by a minimum radius that can beachieved with the process. In FIG. 12 this radius is indicated byR_(ra)m Experiments have shown that an edge radius R_(min) of less than200 nm can be achieved with the fly-cut process on substrates made ofCaF₂.

As a result of the high relative velocity between the cutting edge 24and the substrate 30 a very smooth surface of the microlenses isachieved. Nevertheless this surface is often not perfectly smooth, butdisplays a micro-roughness. This micro-roughness may depend on thematerial properties of the substrate 30 and also on the quality and wearconditions of the tool 22. For example, crystal imperfections of a CaF₂substrate may result in material disruptions of some micrometers indiameter and some nanometers in depth.

In order to reduce such a micro-roughness or local disruptions, thesurface manufactured with the fly-cut machine 20 may be polished in anadditional polishing step. Various polishing methods are suitable forthis purpose, for example methods that involve the use of a polishingcloth or a polishing suspension. It is also envisaged to carry out thepolishing step with the help of a robot or by applying magnetorheologigfluids or fluid jets. Alternatively or additionally, polishing may beachieved with a rotating polishing cylinder having a profile on itsoutside which is a negative of the surface profile of the array. Such apolishing cylinder may be mounted to the spindle 26 of the fly-cutmachine 20. This has the advantage that imperfections with regard to thesurface shape of the microlenses caused by the polishing step areminimized.

It is also possible with the fly-cut process to form microlenses onsubstrates that are coated. For example, a CaF₂ may be coated with asofter material like a resist, another varnish or a metal such as Al orNi. The microlenses are cut directly either into the coating only orboth into the coating and the substrate material. The machined surfaceis then etched in an etch step in order to remove the remaining coating,such as with a constant removing rate.

The fly-cut process has the additional advantage that all microlensesformed with the same tool are substantially identical. This holds trueat least for cases in which material wear of the cutting edge 24 can beneglected during the manufacturing process of a single microlens array.Although the cutting edge 24 may slightly deviate from the ideal shape,all microlenses will nevertheless have the same (non-ideal) shape. Inmany cases this is advantageous because it results in a systematic errorof the intensity distribution obtained in the mask plane.

Such systematic errors can usually be corrected more easily thanuncorrelated errors which are typically for microlens arrays producedwith other manufacturing technologies.

On the other hand, since the microlenses are manufactured individually(or in small groups, as will be explained further below), the fly-cutprocess makes it possible to selectively introduce geometric deviationsamong the microlenses of a single microlens array or even within asingle microlens. By carefully designing these deviations it is possibleto achieve an averaging effect that results in a smoother intensitydistribution in the mask plane. This will be explained in more detailfurther below in section 3.

The fly-cut process is suitable to manufacture all kinds of elongatedmicrolenses, for example microlenses used in optical integrators or inscattering plates. In principle it may also be envisaged to use thefly-cut process to produce arrays of toric microlenses by cutting twoorthogonal sets of cylindrical microlenses on the same side of asubstrate.

In the following certain improvements of the fly-cut process aredescribed in more detail.

2.4 Substrate Orientation

Surface micro-roughness causes scattering and requires increasedexpenditures for polishing the surface. Scattering light generallyresults in a degradation of the system performance.

In unisotropic optical materials, for example CaF₂, the surfaceroughness also depends on the direction along which the cutting edgecuts into the substrate. This cutting direction has to be distinguishedfrom the direction along which the substrate is slowly moved during thefly-cut process.

In the case of CaF₂ the crystal axes are often oriented with respect tothe optical axis such that a {111} crystal plane is arrangedperpendicular to the optical axis. Here the expression {111} denotes aset of equivalent crystal planes using the Miller indices formalism. Incubic crystals such as CaF₂ each {111} plane is perpendicular to one ofthe equivalent crystal directions (111). The round brackets in (111)denote the set of all equivalent crystal directions <111>, <-1-1-1>,<-1-11>, <1-1-1>, <11-1>, <-111>, <1-11> and <11-1>. For details withregard to Miller indices, crystal directions and crystal planesreference is made to US 2004/0105170 A1 assigned to the applicant.

If a CaF₂ crystal is oriented such that a {111} crystal plane isoriented perpendicularly to the optical surface, i.e. parallel to amachining surface of a substrate on which the microlenses are to beformed, the crystal displays a three-fold symmetry with respect to thecutting directions for which optimum results in terms of surfaceroughness are achieved.

FIG. 7 shows a top view on a substrate 30 with a machining surfaceparallel to a {111} crystal plane. Broken lines 60 indicate theintersection of the three equivalent {100} crystal planes with the {111}crystal plane which is parallel to the machining surface of thesubstrate 30. The broken lines 66 form a triangle with corners 68.Arrows 70 indicate ideal cutting directions that are perpendicular tothe three {100} crystal planes. Surprisingly the ideal cuttingdirections are not mere orientations, but are directions in the sensethat optimum results may only be obtained if the cutting edge 24 cutsinto the substrate along the direction of the arrows 70. With any of theopposite cutting directions, which are indicated by FIG. 7 by brokenline arrows 72, the machined surface will become significantly rougher.

If microlenses are to be manufactured only on one side on a {111} CaF₂crystal, the crystalline substrate 30 has simply to be oriented withrespect to the rotating cutting edge 24 such that the cutting edge cutsinto the substrate 30 along any of the three ideal cutting directionsindicated by arrows 70. The turn table 44 may be used for rotating thesubstrate 30 until the ideal angular position is reached.

If two orthogonal arrays of microlenses shall be fabricated on oppositesides of a {111} CaF₂ crystal, as it is the case with the integratormembers 12 and 14, it is not possible to manufacture both arrays withthe ideal cutting direction. In this situation it is possible to make acompromise in which both actual cutting directions are as close aspossible to an ideal cutting direction.

This situation is shown in FIGS. 8 and 9 which show top views on a topsurface and a back surface of the integrator member 12, respectively.The actual cutting directions on both sides correspond to thelongitudinal axes along the microlenses 12Y, 12X extend. As is indicatedin FIGS. 8 and 9, it is possible to obtain a deviation by 15° betweenthe actual cutting direction and the ideal cutting directions 70 forboth surfaces. Since deviations between an ideal cutting direction andan actual cutting direction up to 20° are usually tolerable, themicrolenses 12Y and 12X can still be produced with a small surfaceroughness although none of the microlenses has been fabricated with theideal cutting direction.

In projection exposure apparatus of the scanning type, the illuminatedfield on the mask has usually large dimensions along an X direction andmuch smaller dimensions along a Y direction. This has also implicationsfor the refractive power of the microlenses 12Y, 12X, 14X and 14Y. Thefirst microlenses 12Y and the fourth microlenses 14Y require only asmall refractive power because the illuminated field has a smalldimension along the Y direction. The second microlenses 12X and inparticular the third microlenses 14X desirably have a larger refractivepower because they have to produce light bundles having a larger maximumdivergence angle.

As has been mentioned above, the second microlenses 12X and the thirdmicrolenses 14X have a larger refractive power than the firstmicrolenses 12Y and the fourth microlenses 14Y. This results in anincreased sensibility of the second microlenses 12X and the thirdmicrolenses 14X to fabrication tolerances and surface micro-roughness.Therefore it can be advantageous to produce the second microlenses 12Xand the third microlenses 14X with an ideal cutting direction 70, orwith only some degrees deviation from the ideal cutting direction 70,whereas the less sensible first microlenses 12Y and fourth microlenses14Y are fabricated with larger deviations from an ideal cuttingdirection 70. As has been mentioned above, deviations from the idealcutting direction 70 in excess of about 20° usually result insignificant deteriorations with respect to the surface roughness.However, in many cases such an increased surface roughness is tolerablefor the only slightly curved first microlenses 12Y and the fourthmicrolenses 14Y.

2.5 Surface Profile Manipulation

Often it is desirable to be able to modify the curved surface profile ofthe microlenses produced with the fly-cut process. One incentive may bethat it is difficult to fabricate microlenses having a non-circularsurface profile. This is because the fabrication of a cutting edge 24which is non-circular is a very sophisticated process and may involve,for example, the use of measurement equipment that is able to determinethe curvature and the corrugation of the edge on a 10 nm scale. Incontrast, the fabrication of tools 22 with a circular cutting edge 24 ismuch simpler and less expensive. Therefore it may often be advantageousto manufacture microlenses having a circular surface profile in afly-cut process, and to modify this profile afterwards with othermechanisms.

Another motivation to modify the surface profiles is to selectivelychange the angular distribution of the light impinging on the mask plane18. The angular distribution determines the telecentricity properties ofthe illumination system, for example, and may be modified by changingthe curved surface profile of the third microlenses 14 only in certainareas on the surface of the second integrator members 14.

This is explained with reference to FIG. 10 which shows a top view ofthe second integrator members 14. For the sake of clarity only some ofthe third microlenses 14X are shown in the upper left corner. The curvedsurface profile of the third microlenses 14X is modified in areas 72such that more light energy is concentrated in the center of the lightbundles emitted by the microlenses 14X in the areas 72. As a result,more light energy concentrates in the center of the mask plane 18, as itis indicated in FIG. 10 by continuous lines within the areas 72. Such amodified angular distribution may be produced by flattening the vertexzones of the third microlenses 14X.

In second areas 74 the curved surface profiles of the third microlenses14X are modified in a different way.

Here the modification is carried out such that more light energy isrefracted into larger angles, which results in an intensity distributionin the mask plane 18 where light energy is shifted from the center tothe margins of the illuminated field in the X direction (see continuouslines in areas 74). Such a modification may be carried out by removingmaterial such that the curvature decreases towards the lateral sides ofthe third microlenses 14X.

Since the first areas 72 and the second areas 74 form two pairs ofopposite poles, with the pairs being arranged perpendicularly to eachother, a point in the center of the mask plane 18 receives more lightfrom the directions associated with the first areas 72 and less lightenergy from the directions associated with the second areas 74. For apoint at the margins (in X direction) of the illuminated field the sameapplies vice versa. This exploits the fact that for each light ray thereis a relationship between the angle of incidence on the mask plane 18 onthe one hand and the distance form the optical axis OA at which thelight ray passes through the second integrator member 14 on the otherhand (see also FIG. 2). As a result of the complementary intensitydistributions produced by the microlenses within areas 72 and areas 74,each point on in the mask plane nevertheless receives the same amount oflight energy.

As a matter of course, the particular modification shown in FIG. 10 isonly exemplary. Various other modifications of the surface profile ofthe microlenses, which are restricted to certain areas on the secondintegrator member 14, are possible for a field dependent modification ofthe angular distribution. For example, the microlenses in the areas 72,74 may produce different angular intensity distributions, or the numberand/or position of the areas may be varied. As a matter or course, thesurface profile of all microlenses 14X may be modified to differentdegrees as well. Thus surface profile modification restricted to certainareas of, or varying over, the total surface of the microlenses 14X isan excellent measure to selectively produce field dependentmodifications of the angular distribution. The angular distributiondirectly determines the telecentricity and what is usually referred toas pupil ellipticity of the illumination sys-tern.

Yet another motivation to selectively modify the surface profiles is toimprove the intensity uniformity in the mask plane. This will beexplained in more detail further below in section 3.3.

In the following various methods are described that make it possible toselectively modify the curved surface profile of microlenses producedwith a fly-cut process.

2.5.1. Polishing

The polishing methods described above in section 2.3 can be used tomodify the curved surface profile of all microlenses of an array, forexample in order to achieve the effect of an aspheric lens in a planeperpendicular to the longitudinal axes of the microlenses, or onlywithin certain areas of the array. For example, with a polishing clothit is possible to flatten the vertex zones of the microlenses. Withother polishing methods, for example magnetorheologig fluids, it is evenpossible to abrade material selectively from the lateral sides of themicrolenses.

2.5.2 Ion Beams

If the microlenses are exposed to a homogeneous ion beam, a plurality ofions impinges on the surface of the microlenses or portions thereof. Theions remove atoms from the surface of the microlenses with an abrasionor etch rate R that depends, among others quantities, on the material ofthe microlenses and also on the energy and current density of the ions.Furthermore, the etch rate R is a strong function of the local angle ofincidence.

The graph shown in FIG. 11 illustrates, for an ion energy of 500 eV, thedependency of the etch rate R on the local angle of incidence φ for CaF₂(continuous line) and, for comparison, a resist (broken line).

The cross section of the ion beam is selected such that its maximumdimension is greater, such as by a factor of 5 or more, than the pitchof the microlenses. Since the ions move (at least substantially) allparallel to a common ion beam direction, this condition ensures that thelocal angle of incidence φ, at which the ions impinge on the surface,varies along a direction which is perpendicular to the longitudinal axesof the microlenses. This becomes clear from FIG. 12 which shows aplurality of third microlenses 14X in a perspective view. In thisembodiment an ion beam source 76 produces a homogeneous ion beam havingan ion beam direction 78. The term “homogeneous” in the respect denotesthe fact that the ion current density of the ion beam is (at leastsubstantially) constant over the cross section of the beam. If the ioncurrent density of the ion beam produced by the ion beam source 76significantly falls off towards the outer portions of the beam, adiaphragm may be used as an ion stop that blocks those portions. The ionbeam source 76 is capable of being rotated around two orthogonal axes80, 82 so that the ion beam may be directed under any arbitrary angleonto the third microlenses 14X. If only microlenses shall be etched thatextend in a common plane, it is usually simpler to rotate the substrate30 using a tilt table, which holds the substrate 30, instead of rotatingthe ion beam source 76.

The substrate 30 with the third microlenses 14X formed on top is fixedto a positioning table that makes it possible to displace themicrolenses 14X along orthogonal directions X and Y, as it is indicatedby double arrows 84 and 86, respectively. In this way the area 87 on thethird microlenses 14X exposed to the ion beam may be varied. As a matterof course, the exposed area 87 may have any geometry and may be eithersmaller or larger than is schematically shown in FIG. 12. For example,if the diameter of the ion beam is larger than the total surface of thesecond integrator member 14, all third microlenses 14X aresimultaneously exposed to the ion beam at a given time. If the crosssection of the ion beam has a maximum dimension which is smaller thanthe total surface of the second integrator, only a portion of thissurface is simultaneously exposed to the ion beam. This may beadvantageous if different areas of the third microlenses 14X shall bereworked differently, as has been explained above under section 2.5 withreference to FIG. 10. This may be used to remove ripples in theintensity distribution in the mask plane by exploiting an averagingeffect, as will be explained in more detail below.

If α denotes the aperture angle formed between the ion beam directionand the Z direction, and β denotes the azimuth angle between the ionbeam direction 78 and the Y direction (see FIG. 13), the local angle ofincidence φ on the curved surface of the microlenses 14X is given byequation (1):

${\cos (\phi)} = \frac{{\cos (\alpha)} - {{\sin (\alpha)}{\sin (\beta)}{f^{\prime}(x)}}}{\sqrt{1 + f^{\prime \; 2}}(x)}$

The function f(x) defines the curved surface profile of the microlenses.

If an ion beam is directed onto one or a plurality of microlenses, thedependency of the etch rate R on the local angle of incidence φ, andalso the dependency of this angle φ on the surface profile f(x), ensuresthat the etch rate R varies over the surface of the microlenses. This,in turn, makes it possible to modify the surface profile of themicrolenses in one go. In particular, there is no need to direct a verythin ion beam on a portion of a single microlens, and to vary the etchrate by controlling the etch time. Instead the surface profilemodification is mainly determined by the angles α and β under which theion beam is directed onto the microlenses 14X.

Since the etch rate R also depends on the material on which the ionimpinge (see FIG. 11), a coating consisting of a carefully selectedmaterial, or including layers of different materials, may be depositedon the third microlenses 14X. The material of the coating then providesan additional degree of design freedom that may be exploited when theprocess parameters are determined that are used for obtaining thedesired surface manipulation. If the material is an anisotropic crystalsuch as CaF₂, the etch rate R also depends on the crystal orientation.This should be considered when determining the parameters for the ionetch process.

FIG. 14 shows a first exemplary setup in which the azimuth angle β=0°.The ion beam direction 78 then lies in a plane of incidence which isparallel to the Y direction and perpendicular to the XY plane in whichthe third microlenses 14X extend. Such a setup ensures that the abrasioncaused by the ion beam is symmetrical on both sides of each thirdmicrolens 14X. However, with such a setup it is difficult, even withlarger aperture angles α, to obtain a significant abrasion on the vertexzone of each microlens. This is illustrated in the graph of FIG. 15which shows the dependency of the etch rate R over the pitch of a singlemicrolens for different aperture angles α. With the exception of theaperture angle α=75°, the etch rate R increases with increasing distanceX from the vertex line of the microlens in the X direction.

Thus this setup with an azimuth angle β=0 is mainly suitable for surfaceprofile modifications in which material shall be removed mainly from thelateral sides of the microlenses. Usually such a modification gives theprofile a more triangular shape.

Often, however, it is desired to flatten the vertex zone in order togive a surface profile a more rectangular shape. In these cases it isbetter to use the second exemplary setup shown in FIG. 16 in which theazimuth angle β=90°. The ion beam direction 78 is then in planes ofincidence perpendicular to the Y direction.

As is shown in the graph of FIG. 17, the etch rate R now varies morestrongly, and with a large aperture angle α=75° the maximum etch rate Ris achieved almost at the vertex line of the microlens (x=0). However,for aperture angles OC≠0° the etch rate R is not symmetrical withrespect to the vertex line (x=0). This means that the surface profile ofthe third microlenses 14X would be deformed asymmetrically with respectto the vertex lines. Since this is, at least generally, not desirable,an additional ion beam etching process has to be carried out with an ionbeam having the same azimuth angle β=90°, but with an aperture angle α.In FIG. 16 ion beam directions 78 a, 78 b of two such ion beams havingopposite aperture angles α are indicated as dotted lines.

An exposure of the microlenses to two ion beams having different ionbeam directions 78 a, 78 b may be carried out subsequently with a singleion beam source 76 that is tilted or otherwise adjusted between the twoetching processes. The same effect is obtained with the help of anadditional ion beam source which is appropriately arranged above thethird microlenses 14X. The provision of two ion beam sources makes itpossible to achieve a reduction of the machining time by a factor oftwo.

The graph of FIG. 18 shows the total etch rate R which is obtained iftwo ion beams, are directed from both sides onto the third microlenses14X. The total etch rate R is now symmetrical also for aperture anglesα≠0°, and for larger aperture angles α>50° the etch rate R is greater onthe vertex lines of the microlenses than further away from the vertexlines. Thus this setup with an azimuth angle β=90° will usually bedesired if microlenses shall be flattened at their vertex zones.

It is also possible to vary the aperture angel α during the exposure ofthe microlenses. This provides an additional degree of design freedomthat may be exploited when the process parameters are determined thatare used for obtaining the desired surface manipulation.

It should be noted that the foregoing description of the ion beametching process exemplarily relates to the third microlenses 14X. As amatter of course, other microlenses, and in particular the secondmicrolenses 12X, may be reworked using ion beams in the same manner ashas been described above.

3. Geometry Variations

In the following certain variations of the geometry of the microlensesare described. The variations, which may be produced using the fly-cutprocess, are intended to partly destroy the strict geometric correlationbetween the second and third microlenses 12X, 14X. If not all secondmicrolenses 12X illuminate the corresponding third microlenses 14X inexactly the same manner, the intensity distributions produced by thethird microlenses 14X in the mask plane 18 will be slightly shiftedalong the X direction. This results in an improved uniformity of theintensity distribution in the mask plane, as will be explained in detailbelow.

Of course, she same approach may also be applied, mutatis mutandis, forthe first and fourth microlenses 12Y, 14Y. However, the uniformity ofthe intensity distribution along the Y direction is often of littleconcern since the scanning movement of the mask produces an averagingeffect anyway. For that reason the following description always relatesto the second and third microlenses 12X, 14X that produce the intensitydistribution along the X direction.

3.1 Pitch Variation

FIG. 19 shows an enlarged cross section through first and secondintegrator members 112, 114. In this embodiment it is assumed that thesecond microlenses 112X of the first integrator member 112 are regularlyarranged with equal pitches pi and all have the same surface profile.The third microlenses 114X arranged on the second integrator member 114have circular surface profiles with the same radius of curvature.However, the pitches p₂₁, p₂₂, . . . p_(2n) of the third microlenses114X vary. In this embodiment the variation of the pitches p₂₁, p₂₂, . .. p_(2n) follows an equipartition function so that all pitches p₂₁, p₂₂,. . . p_(2n) within a certain range occur with the same occurrence.Since the mean value of the pitches p₂₁, p₂₂, p_(2n) is the same as thepitch pi of the first integrator member 12, there is nevertheless a oneto one correspondence between opposing second and third microlenses112X, 114X. As a result of the pitch variation, vertex lines V of thesecond microlenses 112X generally do not coincide any more with vertexlines V₁, V₂, . . . , V_(n) of the corresponding third microlenses 114X.Instead, the distance between corresponding vertex lines varies also inaccordance with an equipartition function.

FIG. 20 is a graph showing an intensity distribution E(x) as a functionof the x coordinate in the mask plane 18 produced by a single thirdmicrolens 114X. The surface profile of the third microlenses 114X isdetermined such that the intensity distribution in the mask plane 18along the x direction is, at least within the illuminated field (i.e. xcoordinate between −x₀ and +x_(o)), almost flat.

However, for various reasons, such as a residual surface roughness ofthe microlenses 114X, the intensity distribution is not perfectly flat,but displays a plurality of ripples having small amplitudes and a highfrequency, as it is schematically indicated in FIG. 20. If all thirdmicrolenses are assumed to produce the same intensity distribution asshown in FIG. 20, a superimposed intensity distribution would result ina total intensity distribution in which the ripples are, in absoluteterms, due to the superposition even stronger than for a single thirdmicrolens. Due to manufacturing tolerances, however, the thirdmicrolenses will never be perfectly equal, and consequently theintensity distributions produced by the third microlenses in the maskplane 18 also slightly vary. The superposition of such slightly varyingintensity distributions results in an averaging or smearing effect thatpartly eliminates the ripples observed in the intensity distribution ofeach single third microlens 114X.

However, larger ripples that may be produced by flaws in or wear of thecutting edge 24, for example, may be present in all third microlenses,and the resulting ripples in the total intensity distribution of allthird microlenses in the mask plane 18 will be clearly detectable. Evenwithout larger ripples, the averaging effect may not be sufficient tosufficiently eliminate the ripples in the total intensity distributionof all third microlenses.

Since the pitches p₂₁, p₂₂, . . . p_(2n) of the third microlenses 114Xvary in the embodiment shown in FIG. 19, the distance betweencorresponding vertex lines varies as well, as has been explained above.As a result, the third microlenses 114X are illuminated by the secondmicrolenses 12X in different ways, namely with illumination bundles thatare laterally displaced along the X direction to different extents.This, in turn, will cause the intensity distributions produced by eachthird microlens 114X to be shifted along the X direction by a slightdistance. This distance is proportional to the distance by which avertex line V₂k of a third microlens 114X is laterally displaced withrespect to the vertex line V of the corresponding second microlens 112X.

In FIG. 21 a plurality of intensity distributions as shown in FIG. 20are indicated by dotted lines. The intensity distributions are displacedalong the X direction by distances which are given by an equipartitionfunction. Thus each distance is, within a given range, present with thesame occurrence. The strong continuous line indicated by 90 representsthe total intensity distribution obtained by superimposing all intensitydistributions produced by the individual third microlenses 114X. It canbe seen that, due to the averaging effect of the slightly displacedintensity distributions, the total intensity distribution 90 is nowalmost completely flat within the illuminated field (x coordinatebetween −x_(o) and +x_(o)). As a result of the lateral displacement ofthe intensity distributions in the mask plane 18, some light is lost.This is represented in FIG. 21 by the broader slopes of the totalintensity distribution 90 outside the illuminated field. This loss oflight may be compensated for by a slightly increased light energy pertime unit produced by the light source of the illumination system, forexample.

As a matter of course, the same effect is achieved if not the thirdmicrolenses 114X, but the second microlenses 112X have varying pitches.Furthermore, it is possible that both the second and the thirdmicrolenses 112X, 114X have a varying pitch. For the sake of simplicitythe following description how microlenses with varying pitches may beproduced only refers to the third microlenses 114X.

In the following it is described how third microlenses 114X havingvarying pitches may be produced using the fly-cut process. FIGS. 22 a to22 l show a cross section through the substrate 30 at various subsequentsteps during the manufacture of the third microlenses 114X. It isassumed that the substrate 30 moves along a direction which isperpendicular to cutting edge 24 during these steps.

In FIG. 22 a it is indicated how the tool 22 with its cutting edge 24moves towards the substrate 30. In FIG. 22 b it can be seen how the tool22 cuts into the substrate 30 when it dips into the latter during therotation around the rotational axis 28. FIG. 22 c shows the substrate 30with material removed by the tool 22 so that a first microlens 114X1 isformed.

After the first microlens 114X1 has been completely formed, thesubstrate 30 is displaced sideward so that the next microlens can beformed which is arranged side by side to the microlens 114X1.

FIGS. 22 e and 22 f correspond to the FIGS. 22 b and 22 c. In FIG. 22 fit can be seen that a second microlens 114X2 has been formed which hasthe same pitch as the first microlens 114X1.

After the second microlens 114X2 has been formed, the substrate 30 isagain displaced sideward, but now by a smaller distance. As a result thetool 22 partly cuts into the second microlens 114X2 which results in aW-shaped groove 92 formed between the second microlens 114X2 and thethird microlens 114X3.

After the manufacture of the third microlens 114X3 is completed, thesubstrate 30 is displaced again sideward, but now by a distance which islarger than the distances before. As a result, the fourth microlens114X4 produced by the tool 22 is now spaced apart from the thirdmicrolens 114X3 by a ridge 94 having a triangular profile.

From FIGS. 22 a to 22 l it becomes clear that the manufacture ofmicrolenses having varied pitches produces undesired grooves 92 orridges 94 between adjacent microlenses that could, in principle,deteriorate the performance of the microlens array. However, for thesake of clarity the grooves 92 and the ridges 94 are significantlyenlarged. In a real microlens array the pitch variations are very smalland may not exceed 1% of the average pitch. Consequently, the grooves 92and the ridges 94 are so small that any adverse effects on the opticalproperties are significantly offset by the improvements obtained by thesuperposition of slightly displaced intensity distributions in the maskplane 18.

In this context it should be noted that at the edges where adjacentmicrolenses abut there are differences to the ideal shape anyway, as hasbeen explained above with reference to FIG. 12.

3.2 Longitudinal Axis Variation

FIGS. 23 and 24 show an array of third microlenses 214X according toanother embodiment in a top view and a side view, respectively. In thisembodiment the third microlenses 214X extend parallel to each other, buttheir longitudinal axes are inclined with respect to the longitudinalaxes of the second microlenses 12X. The regular array of secondmicrolenses 12X is indicated in FIG. 24 by dotted borderlines 96 whereadjacent second microlenses 12X abut.

The third microlenses 214X may be considered as including a plurality ofdifferent portions that are aligned one behind the other. The portionshave longitudinal axes and vertex lines that form zigzag lines. Sinceall third microlenses 214X are arranged in parallel, which implies anequal pitch for all microlenses along their entire length, alsoborderlines 97 where adjacent third microlenses 214X abut form zigzaglines. As can be clearly seen, the zigzag borderlines 97 of the thirdmicrolenses 214X “wind” around the straight borderlines 96 of thecorresponding second microlenses 12X.

The same also applies for the vertex lines of the third microlenses 214Xwith respect to the vertex lines of the second microlenses 12X.Therefore the borderlines 96 and 97 shown in FIG. 23 may equallyrepresent vertex lines of a microlens array that is laterally displacedby one half of the microlens pitch. The top view of FIG. 23 may then beregarded as a projection of the vertex lines of the second microlenses12X and the third microlenses 214X along the optical axis (i.e. Zdirection) of the optical integrator 10.

This zigzag configuration ensures the one to one correspondence betweenthe second microlenses 12X and the third microlenses 214X is maintained.Since the zigzag line is regular with an inclination angle of δ, thelateral shift of the vertex lines of the third microlenses 214X withrespect to the vertex lines of the corresponding second microlenses 12Xalso follows an equipartition function. Due to the lateral displacementsof the third microlenses 214X with respect to the second microlenses12X, the third microlenses 214X are illuminated by the secondmicrolenses 12X in different ways, namely with illumination bundles thatare laterally displaced along the X direction to different extents. Theextent of the displacement continuously varies along the Y direction foreach single pair of corresponding microlenses. This, in turn, will causethe intensity distributions produced by each third microlens 214X to beshifted along the X direction by a slight distance. Thus essentially thesame effect is achieved as it is shown in FIG. 21 for the thirdmicrolenses 114X of the embodiment shown in FIG. 19.

As a matter of course, a similar effect is achieved if the axes of thethird microlenses do not form zigzag lines, but serpentines (i.e.continuously curved lines), or zigzag lines having a more complicatedform than shown in FIG. 23. Furthermore it should be noted that also inthis embodiment only the second microlenses 12X, or both the second andthird microlenses 12X, 214X, may have adjacent portions withlongitudinal axes that form zigzag lines or serpentines of variousshapes.

For manufacturing such microlenses the fly-cut process may be used aswell. For example, for producing microlenses having a serpentinelongitudinal axis, it is only slow rotate of the turn table 44 aroundthe rotational axis 46 during the fly-cut process may be involved.

3.3 Surface Profile Variation

It has already been mentioned that the profile shape modificationsdescribed above in section 2.5 may advantageously be used also forimproving the uniformity of the intensity distribution in the mask plane18. By polishing or ion beam etching individual microlenses or portionsthereof, it is possible to selectively produce surface profilevariations that increase the variations that are already present due tofabrication tolerances. By carefully designing these variations it ispossible to increase the averaging effect obtained by the superpositionof different intensity distributions in the mask plane 18 to such anextent that an almost perfectly uniform intensity distribution isachieved.

In the following alternative methods are explained how the surfaceprofile may be selectively varied with very simple mechanisms.

3.3.1 Interleaved Microlens Manufacturing Sequence

During the manufacture of a microlens array the cutting tool 22 isusually subject to wear which affects the geometry and cutting qualityof the cutting edge 24. Usually the microlenses are manufactured oneafter the other so that a new microlens is manufactured immediatelyadjacent an already existing one. Then the surface profiles of adjacentmicrolenses are almost identical, but will vary slightly over the entiresurface of the array due to the increasing wear of the cutting tool 22.This introduces a systematic error in the uniformity distribution whichcannot be easily corrected by other mechanisms.

These adverse effects can be avoided by using a manufacturing process inwhich the microlenses are manufactured in an interleaved manufacturingsequence. What this means is illustrated in FIGS. 25 a to 25 e whichshow the substrate 30 and the tool 22 in cross sections at varioussubsequent stages during the manufacturing process.

After the first microlens 314X11 has been formed (FIG. 25 a), thesubstrate 30 is moved sideward by a multiple of microlens pitches (FIG.25 b) before the second microlens 314X12 is formed. The same appliesalso for the third microlens 314X13 after the manufacture of the secondmicrolens 314X12 has been completed (FIG. 25 c), etc.

After the entire surface of the substrate has been covered withmicrolenses 314X11, 314X12, 314X13, . . . , 314X1 n spaced apart by amultiple of the microlens pitch, a second array of equally spacedmicrolenses 314X21, 314X22, 314X23, 314X2 n is manufactured in the samemanner, as is shown in FIGS. 25 d and 25 e.

This interleaved manufacturing sequence ensures that the differencesbetween the surface profiles of adjacent microlenses may besignificantly enough to achieve an averaging effect similar to what hasbeen explained above in sections 3.1 and 3.2. Nevertheless there is nosystematic change of the surface profile in a direction perpendicular tothe longitudinal direction of the microlenses.

3.3.2 Reworking with Cutting Tool

After the complete array of microlenses has been manufactured on thesubstrate 30, some, for example every second or every third, microlensmay be reworked using the same or desirably a slightly different cuttingtool 22, thereby producing a surface profile variation.

FIG. 26 illustrates how every second of third microlenses 412X isreshaped with a tool 122 which is different from the tool that formedthe third microlenses 412X in a first run. As a matter of course, themicrolenses 412X may be reworked also with more than one additional tool122.

FIG. 27 illustrates an alternative method in which all or only some ofthe microlenses 512X are reworked with the same tool 22 that has beenused for the manufacture of the microlenses 512X in the first run.However, the substrate 28 is positioned with respect to the spindle 26such that the microlens to be reworked is slightly laterally offset withrespect to the tool 22. This results in pitch variations similar tothose described above with reference to FIG. 19.

3.3.3 Different Tools

A very simple way to produce microlenses having different surfaceprofiles or pitches is to use different cutting tools that arespecifically adapted to the desired surface profile and/or pitch. FIG.29 shows a cross section through a microlens array in which thirdmicrolenses 614X all have the same pitch, but different radii ofcurvature which are denoted by r₁ to r₄. The different curvatures aresignificantly exaggerated in FIG. 29 for the sake of clarity; in realmicrolens arrays the radius of curvature variations may be as small asone percent. If such an array of microlenses 614X is illuminated by aregular array of second microlenses 12X, four intensity distributionshaving different widths will be superimposed in the mask plane 18. Thiswill significantly reduce undesired ripples in the overall intensitydistribution.

For manufacturing the third microlenses 614X, the interleavedmanufacturing sequence explained above in section 3.3.1 with referenceto FIGS. 25 a to 25 e can be applied, wherein each array of parallel andspaced apart microlenses is formed with a different cutting tool 22.

3.3.2 Multi-Edge Tool

Instead of using a tool having only a single cutting edge, a tool may beused on which several cutting edges are arranged side by side. The morecutting edges are provided on the multi-edge tool, the shorter will bethe total machining time for manufacturing the microlens array.

The provision of a multi-edge tool makes it possible to produce in asingle fly-cut cycle a plurality of microlenses having different surfaceprofiles and/or pitches. For example, for manufacturing the thirdmicrolenses 614X shown in FIG. 28 a four-edge cutting tool may be usedthat forms four adjacent microlenses 614X in one fly-cut cycle.

FIG. 29 shows a schematic front view of a tool 222 including twodifferent cutting edges 224 a, 224 b. Here the cutting edges 24 a, 24 bare spaced apart by one microlens pitch. Such an arrangement may beadvantageous for reworking an array of microlenses in a way similar towhat is shown in FIG. 21.

FIG. 30 is a front view of a multi-edge tool 322 having three adjacentcutting edges 324 a, 324 b, 324 c. The multi-edge tool 322 is optimizedfor manufacturing a periodic array of groups of adjacent microlenses,wherein the microlenses within one group have different surfaceprofiles. With the tool 322 it is possible that no tool changes areinvolved during the manufacturing process.

4. Scattering Plates

As has been mentioned above, the fly-cut process with its differentembodiments described in sections 2 and 3 may also be advantageouslyused for the production of other kinds of elongated microlenses. Forexample, international patent application PCT/EP2007/001267, whose fulldisclosure is incorporated herewith by reference, proposes to usescattering plates having parallel arrays of microlenses in illuminationsystems of microlithographic projection exposure apparatus.

FIG. 31 is a perspective view of the optical integrator 10 in arepresentation similar to FIG. 1. In this embodiment two scatteringplates 98 and 100 are arranged in front of and behind the opticalintegrator 10, respectively. The scattering plates 98, 100 also containarrays of microlenses that have varying geometries similar to themicrolenses that have been described above.

The above description has been given by way of example. From thedisclosure given, those skilled in the art will not only understand thepresent disclosure and its attendant advantages, but will also findapparent various changes and modifications to the structures and methodsdisclosed. The applicant seeks, therefore, to cover all such changes andmodifications as fall within the spirit and scope of the disclosure, asdefined by the appended claims, and equivalents thereof.

1. A method of manufacturing an array of elongated microlensesconfigured to be used in an illumination system of a microlithographicprojection exposure apparatus, the method comprising: a) repeatedlymoving a cutting tool comprising a cutting edge relative to a substratein a fly-cut process so that the cutting edge cuts into the substrate;and b) moving the substrate during step a) along a longitudinaldirection; c) moving the substrate at least substantially perpendicularto the longitudinal direction; and d) repeating a) and b) to provide thearray of elongated microlenses, wherein the elongated microlenses havelongitudinal axes which are parallel to the longitudinal direction. 2.The method of claim 1, comprising moving the substrate parallel to amachining plane.
 3. The method of claim 1, comprising moving thesubstrate along straight parallel lines during b).
 4. The method ofclaim 1, comprising moving the substrate along straight zig zag lines orserpentines during b).
 5. The method of claim 1, comprising rotating thecutting tool around a rotational axis.
 6. The method of claim 5, whereinthe rotational axis extends at least substantially parallel to asubstrate surface on which the elongated microlenses are formed.
 7. Themethod of claim 5, wherein the cutting edge points away from therotational axis.
 8. The method of claim 5, wherein the cutting edge isspaced apart from the rotational axis by at least 5 mm.
 9. The method ofclaim 5, wherein the cutting edge is spaced apart from the rotationalaxis by more than 9 mm and less than 11 mm.
 10. The method of claim 1,comprising rotating the cutting tool with a rotational frequency inexcess of 5000 1/min.
 11. The method of claim 1, comprising rotating thecutting tool with a rotational frequency between 8000 1/min and 100001/min.
 12. The method of claim 1, wherein the cutting edge is concavelyor convexly curved.
 13. The method of claim 1, comprising, during c),moving the substrate substantially by a multiple of a pitch of theelongated microlenses.
 14. The method of claim 1, comprising reworking aportion of the surface of an elongated microlens to change a surfaceprofile of the elongated microlens.
 15. The method of claim 14, whereinreworking the portion of the surface of the elongated microlenscomprises repeating a) and b) with an additional cutting tool having adifferent cutting edge.
 16. The method of claim 14, wherein reworkingthe portion of the surface of the elongated microlens comprisesrepeating a) and a) with the cutting tool turned around by 180°.
 17. Themethod of claim 1, comprising polishing a portion of an elongatedmicrolens.
 18. The method of claim 1, comprising exposing a portion ofan elongated microlens to an ion beam.
 19. The method of claim 18,wherein all ions in the ion beam move at least substantially parallel toan ion beam direction.
 20. The method of claim 18, comprising applying acover layer on the portion of the elongated microlens prior to exposureto the ion beam.
 21. The method of claim 18, wherein the ion beam has across section with a maximum dimension which is greater than a pitch ofthe elongated microlens.
 22. The method of claim 18, wherein the ionbeam has a cross section with a maximum dimension which is greater than5 times a pitch of the elongated microlens.
 23. The method of claim 18,wherein an orientation between the portion of the elongated microlensand the ion beam is changed to adjust for a change of a surface profileof the elongated microlens.
 24. The method of claim 18, wherein theelongated microlenses extends parallel to a second direction, and theion beam has a direction which is in a plane of incidence parallel tothe second direction and perpendicular to a plane in which the elongatedmicrolenses extend.
 25. The method of claim 18, wherein the elongatedmicrolenses extend parallel to a second direction, and the ion beam hasa direction which is in a plane of incidence that is perpendicular tothe second direction.
 26. The method of claim 25, comprising exposingthe elongated microlenses to another ion beam which has a direction inthe plane of incidence but symmetrical to a surface normal of thesubstrate.
 27. The method of claim 18, wherein only those portions ofthe elongated microlenses arranged in different areas of the array arereworked.
 28. The method of claim 1, wherein the substrate: is a CaF₂crystal having a surface that is machined by the cutting tool, thesurface of the CaF₂ crystal being at least substantially parallel to a{111} crystal plane, and is oriented with respect to the moving cuttingtool so that the cutting edge cuts into the substrate along an actualcutting direction which forms a deviation angle of less than 20° to anideal cutting direction, the ideal cutting direction being perpendicularto one of the three {100} crystal planes, and the ideal cuttingdirection pointing toward a corner of a triangle formed by theintersection of the {111} crystal plane with the three {100} crystalplanes.
 29. The method of claim 28, wherein two arrays of parallelelongated microlenses are manufactured subsequently on opposite sides ofthe substrate, and the surface of the CaF₂ crystal is at leastsubstantially parallel to a {111} crystal plane during the manufactureof both arrays.
 30. The method of claim 29, wherein the deviation angleis between 13° and 17° for both arrays.
 31. The method of claim 29,wherein the deviation angle equals substantially 15° for both arrays.32. A method of reworking an array of elongated microlenses having amaximum pitch, the array of elongated microlenses being configured foruse in an illumination system of a microlithographic projection exposureapparatus, the method comprising: exposing a portion of an elongatedmicrolens to an ion beam having a cross section with a maximum dimensiongreater than a maximum pitch of the portion of the elongated microlens.33. The method of claim 32, wherein an orientation between the portionof the elongated microlens and the ion beam is changed to adjust for amodification of a surface profile of the elongated microlens by the ionbeam.